The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
As mentioned above, the invention is particularly applicable to a delivery system for delivery of grout to a downhole location within a borehole. Accordingly, the invention will primarily be discussed in relation to that application.
In borehole drilling operations, drilling fluid (commonly referred to as drilling mud) is used for cleaning and cooling a drill bit of a downhole drilling system during the drilling process and for conveying drilling cuttings to the ground surface.
In certain circumstances, an underground area through the borehole is being drilled can be unstable or otherwise vulnerable to the development of fractures through which drilling fluid can escape. The loss of drilling fluid is undesirable, both in economic terms and also as it can lead to a reduction in fluid pressure within the borehole.
With a view to preventing or at least inhibiting the loss of drilling fluid, it is known to deliver grout to the vulnerable location within the borehole in order to seal fractures through which fluid may otherwise escape.
A known grout delivery system is disclosed in WO 2013/078514, the contents of which are incorporated herein by way of reference. With this grout delivery system, grout is formed as a settable mixture of first and second flowable grout material components. The grout delivery system is adapted to be conveyed to a location within the borehole to which the grout is to be delivered in a grouting operation, and to be subsequently retrieved after the grouting operation.
The grout delivery system comprises a delivery head, a first reservoir for receiving a charge of the first grout material component and a second reservoir for receiving a charge of the second grout material component. The delivery system is operable to cause supplies of the first and second grout material components to be conveyed to a mixing zone at the delivery head where they are mixed to form the grout and delivered into the borehole. The first and second reservoirs are configured as chambers of variable volume, whereby volume contract of the chambers causes the first and second grout material components to be expelled therefrom and conveyed to the delivery head. Specifically, each variable volume chamber is defined by a piston and cylinder arrangement, with a piston being selectively moveable within the cylinder to effect volume variation of the chamber. The pistons are responsive to fluid pressure generated within the borehole above the tool assembly, the arrangement being that the fluid pressure acts on the pistons to cause the pistons to move along their respective cylinders, thereby causing volume contraction of the chambers.
The fluid pressure is selectively generated by pumping fluid (typically water) into the drill string above the downhole tool assembly. With this arrangement, water under pressure flows into the tool assembly and acts upon the pistons to cause the pistons to move along their respective cylinders, thereby causing volume contraction of the chambers. This expels grout component material from the reservoirs and causes the expelled material to ultimately flow into the mixing zone, at which the grout component materials mix to react chemically to form the grout. The resulting grout is discharged as a viscous fluid mixture through the outlet and delivered into the borehole. At the completion of the grout delivery process, the delivery of pressurized fluid into the borehole is terminated and the grout delivery system is retrieved by raising it to the ground surface using an overshot assembly attached to a wire line.
In the arrangement disclosed in WO 2013/078514, the chambers are accommodated permanently within the downhole tool assembly and are required to be periodically replenished with grout component material.
In order to facilitate ease of replenishment, it would be desirable for the grout component materials to be contained within a container such a cartridge which can be replaced as necessary when replenishment grout component material is required.
An aspect of the present invention is directed to such an arrangement.
With the arrangement disclosed in WO 2013/078514, it is important that the pistons travel along their respective cylinders in concert (unison) so that appropriate relative proportions of grout component materials are delivered into the mixing zone. If, for any reason, one piston were to advance at a rate different from the other piston, there is a likelihood that the required relative proportions of the grout component materials may be outside of acceptable limits, potentially leading to problems with the resultant grout.
Any tendency for the pistons to advance at different rates would most likely occur at the initial stage of the operation when each piston is required to commence its movement along the respective cylinder. It is at this stage that any tendency for the pistons to resist movement in response to the fluid pressure would be most pronounced. This is the time at which the pistons are most vulnerable to “stick” in the cylinders, thereby disrupting movement of the pistons in concert. Once the pistons have commenced movement along the cylinders there is little likelihood of any “sticking” to disrupt their movement in concert.
Accordingly, it would be advantageous there to be an arrangement in which the two pistons are caused to commence movement in concert in response to fluid pressure at the start of the grouting operation. Once the pistons have commenced to move in concert, there is far less likelihood that either piston would later move in a way in which is not in concert with the other.
A further aspect of the present invention seeks to provide such an arrangement.
With the arrangement disclosed in WO 2013/078514, the downhole tool assembly locates on a landing ring on a downhole drilling assembly already within the borehole. This location establishes a fluid seal whereby fluid (water) can be pumped into the borehole above the downhole tool assembly to generate the fluid pressure as previously described in order to operate the grouting system.
It has been found that this can establish a fluid pressure differential across the fluid seal which at least partially remains even after pumping of fluid (water) under pressure into the borehole has ceased, with the result that the fluid pressure differential can act to resist separation between the downhole tool assembly and the downhole drilling assembly and thereby present difficulties in retrieving the downhole tool assembly.
An aspect of the present invention seeks to address such a difficulty.
It is against this background and the problems and difficulties associated therewith that the present invention has been developed.